1. Field of the Invention
The present invention relates to a toner. In addition, the present invention also relates to an image forming apparatus and an image forming method, which form images using a developer including the toner.
2. Discussion of the Background
Recently, a need exists for high speed image formation in electrophotography. Specifically, super high speed image forming apparatus having a system speed of from 500 to 1700 mm/s are required to stably develop an electrostatic image formed on a photoreceptor and to prevent a toner filming problem in that a toner film (typically a film of a release agent included in the toner but released therefrom) is formed on the photoreceptor, resulting in deterioration of image qualities.
In attempting to fulfill the request, published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 06-202484 (i.e., Japanese patent No. 3128745, i.e., U.S. Pat. No. 5,416,571) discloses a developing device in which at least two developing rollers (backward developing rollers) are arranged in the vicinity of a photoreceptor while rotated in the opposite direction of that of the photoreceptor, and at least one developing roller (forward developing roller) is arranged in the vicinity of a photoreceptor and on an upstream side from the two backward developing rollers relative to the rotating direction of the photoreceptor while rotated in the same direction as that of the photoreceptor. It is described therein that by using such a developing device, image shaving proper image density can be stably produced even at a high speed. However, the toner filming problem is not mentioned therein.
In attempting to solve the toner filming problem, a number of proposals such that a cleaning process is performed on a photoreceptor to remove residual toner particles therefrom after a toner image is transferred from the photoreceptor to an intermediate transfer medium or a receiving material and before the photoreceptor is charged to form the following image have been made. For example, JP-A 2007-132999 discloses a cleaning process using a brush, JP-A 2004-325621 discloses a cleaning process using a blade, and JP-A 09-197932 discloses a cleaning process using a roller. However, particles of the toners used for forming toner images have a variety of forms. Therefore, even when performing such cleaning processes, it is difficult to remove all the toner particles from photoreceptors. In addition, photoreceptors tend to be deteriorated when rubbed by such cleaning members. Therefore, performing a cleaning process using such cleaning members is not a good solution.
In order to solve the toner film problem from the toner side, JP-As 2007-241166, 2007-108622, 2007-78925 and 2006-227190 have proposed toners including a fatty acid metal salt as an external additive thereof. By using a fatty acid metal salt as an external additive, the resultant toners tend to have good filming resistance. However, adding a fatty acid metal salt deteriorates fluidity of the resultant toners. Therefore, it is not proper to use such toners for super high speed image forming apparatus. In addition, conventional fatty acid metal salts have a relatively large particle diameter of not less than 2 μm. Therefore, it is not preferable to use such fatty acid metal salts as external additives.
Because of these reasons, a need exists for a super high speed image forming apparatus, which can produce images with proper image density without causing the toner filming problem.